A barrel, cask, or tun is a hollow cylindrical container, traditionally made of wooden staves bound by wooden or metal hoops. Traditionally, the barrel was a standard size of measure referring to a set capacity or weight of a given commodity. For example, in the UK a barrel of beer refers to a quantity of 36 imperial gallons (160 L; 43 US gal). Wine was shipped in barrels of 119 litres (31 US gal; 26 imp gal).

Modern wooden barrels for wine-making are either made of French common oak (Quercus robur) and white oak (Quercus petraea) or from American white oak (Quercus alba) and have typically these standard sizes: "Bordeaux type" 225 litres (59 US gal; 49 imp gal), "Burgundy type" 228 litres (60 US gal; 50 imp gal) and "Cognac type" 300 litres (79 US gal; 66 imp gal). Modern barrels and casks can also be made of aluminum, stainless steel, and different types of plastic, such as HDPE.

Beverages aged in wooden barrels take on some of the compounds in the barrel, such as vanillin and wood tannins. The presence of these compounds depends on many factors, including the place of origin, how the staves were cut and dried, and the degree of "toast" applied during manufacture. After roughly three years, most of a barrel's flavor compounds have been leached out and it is well on its way to becoming "neutral". Barrels used for aging are typically made of French or American oak, but chestnut and redwood are also used. Some Asian beverages (e.g., Japanese sake) use Japanese cedar, which imparts an unusual, minty-piney flavor. In Peru and Chile, a grape distillate named pisco is either aged in oak or in earthenware.

Some wines are fermented "on barrel," as opposed to in a neutral container like steel or wine-grade HDPE (high-density polyethylene) tanks. Wine can also be fermented in large wooden tanks, which—when open to the atmosphere—are called "open-tops". Other wooden cooperage for storing wine or spirits range from smaller barriques to huge casks, with either elliptical or round heads.

The tastes yielded by French and American species of oak are slightly different, with French oak being subtler, while American oak gives stronger aromas.[1] To retain the desired measure of oak influence, a winery will replace a certain percentage of its barrels every year, although this can vary from 5 to 100%. Some winemakers use "200% new oak", where the wine is put into new oak barrels twice during the aging process. Bulk wines are sometimes more cheaply flavored by soaking oak chips in them instead of being aged in a barrel.

Sherry barrel with special glass barrel head to show barrel's contents; note the layer of flor floating on top of the wine.

Sherry is stored in 600-litre casks made of North American oak, which is slightly more porous than French or Spanish oak. The casks, or butts, are filled five-sixths full, leaving "the space of two fists" empty at the top to allow flor to develop on top of the wine.

Charred white oak barrels are filled with new bourbon whisky and resting in a rack house for a period of typically 4 to 9 years (for good-quality bourbon), with the char giving the bourbon its characteristic copper color.

Laws in several jurisdictions require that whisky be aged in wooden barrels. The law in the United States requires that "straight whisky" (with the exception of corn whisky) must be stored for at least two years in new, charred oak containers.[2] Other forms of whisky aged in used barrels cannot be called "straight".[2]

International laws require any whisky bearing the label "Scotch" to be distilled in Scotland and matured for a minimum of three years and one day in oak casks.[3]

By Canadian law,[4]Canadian whiskies must "be aged in small wood for not less than three years", and "small wood" is defined as a wood barrel not exceeding 700 L capacity.

Since the U.S. law requires the use of new barrels, which is not typically considered necessary elsewhere, whisky made elsewhere is usually aged in used barrels that previously contained American whisky (usually bourbon whisky). The typical bourbon barrel is 53 US gallons (200 L) in size, which is thus the de facto standard whisky barrel size worldwide.[5][6] Some distillers transfer their whisky into different barrels to "finish" or add qualities to the final product. These finishing barrels frequently aged a different spirit (such as rum) or wine. Other distillers, particularly those producing Scotch, often disassemble 5 used bourbon barrels and reassemble them into 4 casks with different barrel ends for aging Scotch, creating a type of cask referred to as a hogshead.[7]

Maturing is very important for a good brandy, which is typically aged in oak casks. The wood used for those barrels is selected because of its ability to transfer certain aromas to the spirit. Cognac is aged only in oak casks made from wood from the Forest of Tronçais and more often from the Limousin forests.

Some types of tequila are aged in oak barrels to mellow its flavor. "Reposado" tequila is aged for up to one year, "Añejo" tequila is aged for up to three years, and "Extra Añejo" tequila is aged for at least three years. Like with other spirits, longer aging results in a more pronounced flavor.

Beers are sometimes aged in barrels which were previously used for maturing wines or spirits. This is most common in darker beers such as stout, which is sometimes aged in oak barrels identical to those used for whisky. Whisky distiller Jameson notably purchases barrels used by Franciscan Well brewery for their Shandon Stout to produce a whisky branded as "Jameson Caskmates". Cask ale is aged in the barrel (usually steel) for a short time before serving. Extensive barrel aging is required of many sour beers.

Vernorsginger ale is marketed as having a "barrel-aged" flavor, and the syrup used to produce the beverage was originally aged in oak barrels when first manufactured in the 19th century.[8] Whether the syrup continues to be aged in oak is unclear.[8]

"Angels' share" is a term for the portion (share) of a wine or distilled spirit's volume that is lost to evaporation during aging in oak barrels. The ambient humidity tends to affect the composition of this share. Drier conditions tend to make the barrels evaporate more water, strengthening the spirit. However, in higher humidities, more alcohol than water will evaporate, therefore reducing the alcoholic strength of the product. This alcoholic evaporate encourages the growth of a darkly colored fungus, the angels' share fungus, Baudoinia compniacensis, which tends to appear on the exterior surfaces of most things in the immediate area.[9][10]

"Devil's cut" is a term for the portion ("cut" is a slang term for a share, particularly of a forced nature) of a wine or distilled spirit's volume that is absorbed into the wooden barrel. Because evaporation is not involved, the strength of the product is not affected. The amount of product loss is determined by the type and quality of the wood and the ambient humidity. Some wood used in barrels can be more or less porous than other wood; more porous woods allow more product to be absorbed. Meanwhile, humid conditions tend to make the barrel absorb that humidity, allowing less product to be absorbed.

If an aging barrel is repurposed and used to store or age another product, the "devil's cut" may in turn be leached into the new product, providing finishing characteristics desired by the barrel user.

This does not occur in steel barrels because steel is entirely non-porous and does not absorb any of the spirit.

Earlier, another size of whisky barrel was the most common size; this was the 40 US gallons (33.3 imp gal; 151.4 L) barrel for proof spirits, which was of the same volume as 5 US bushels. However, by 1866, the oil barrel was standardized at 42 US gallons.

Oil has not been shipped in barrels[11] since the introduction of oil tankers, but the 42-US-gallon size is still used as a unit for measurement, pricing, and in tax and regulatory codes. Each barrel is refined into about 19.74 US gallons (16.44 imp gal; 74.7 L) of gasoline,[12] the rest becoming other products such as jet fuel and heating oil, using fractional distillation.[13]

Barrels have a convex shape and bulge at their center, called bilge. This facilitates rolling a well-built wooden barrel on its side and change directions with little friction, compared to a cylinder. It also helps to distribute stress evenly in the material by making the container more curved[citation needed]. Barrels have reinforced edges to enable safe displacement by rolling them at an angle (in addition to rolling on their sides as described).

A half-completed beer barrel; in wine barrel cooperage this set up is called "mise en rose"

Casks used for ale or beer have shives and keystones in their openings. Before serving the beer, a spile is hammered into the shive and a tap into the keystone.

The wooden parts that make up a barrel are called staves, while the rings that hold them together are called hoops. The latter are generally made of galvanized iron, though historically they were made of flexible bits of wood called withies. While wooden hoops could require barrels to be "fully hooped", with hoops stacked tightly together along the entire top and bottom third of a barrel, iron-hooped barrels only require a few hoops on each end.

Wine barrels typically come in two hoop configurations. An American barrel features 6 hoops, from top to center: head- or chime hoop, quarter hoop and bilge hoop (times two), while a French barrel features 8, including a so-called French hoop, located between the quarter- and bilge hoops (see "wine barrel parts" illustration). The opening at the center of a barrel is called bung hole and the stopper used to seal it bung. The latter is generally made of white silicone.

A barrel is one of several units of volume, with dry barrels, fluid barrels (UK beer barrel, US beer barrel), oil barrel, etc. The volume of some barrel units is double others, with various volumes in the range of about 100–200 litres (22–44 imp gal; 26–53 US gal).

Pre-1824 definitions continued to be used in the US, the wine gallon of 231 cubic inches being the standard gallon for liquids (the corn gallon of 268.8 cubic inches for solids). In Britain, the wine gallon was replaced by the imperial gallon. The tierce later became the petrol barrel. The tun was originally 256 gallons, which explains from where the quarter, 8 bushels or 64 (wine) gallons, comes.

Although it is common to refer to draught beer containers of any size as barrels, in the UK this is strictly correct only if the container holds 36 imperial gallons. The terms "keg" and "cask" refer to containers of any size, the distinction being that kegs are used for beers intended to be served using external gas cylinders. Cask ales undergo part of their fermentation process in their containers, called casks.

Casks are available in several sizes, and it is common to refer to "a firkin" or "a kil" (kilderkin) instead of a cask.

The modern US beer barrel is 31 US gallons (117.34777 L), half a gallon less than the traditional wine barrel. (26 U.S.C. §5051[16])

Barrels are also used as a unit of measurement for dry goods (dry groceries), such as flour or produce. Traditionally, a barrel is 196 pounds (89 kg) of flour (wheat or rye), with other substances such as pork subject to more local variation. In modern times, produce barrels for all dry goods, excepting cranberries, contain 7,056 cubic inches, about 115.627 L.[17]